1. Field of the Invention
The present invention relates to an image forming system having a sheet hold-down device, such as a copying machine, printer and the like, and more particularly it relates to an image forming system having a deep ejection tray capable of collecting a large number of sheets.
2. Related Background Art
Recently, in image forming systems such as copying machines, printers and the like, in order to save time for supplying sheets for a large number of prints, the stacking capacity for sheets to be supplied has been increased. For example, in a large-sized or middle-sized copying machine, a sheet supply deck having a large sheet stacking capacity has been used, and, even in a small-sized copying machine, a sheet supply cassette having a stacking capacity capable of supplying 500 or more sheets at a time has been used.
Pursuant to the increase in the sheet stacking capacity, the sheet collecting capacity has been increased to stack a large number of ejected sheets for copying a large number of prints by using a deeper ejection tray arranged at an upper part of the system.
If the deep ejection tray is used to increase the sheet collecting capacity, when a sheet ejected by a pair of ejector rollers is rested on the ejection tray, the distance that a trailing end of the sheet is dropped onto the ejection tray in space will be increased. Consequently, there is the discrepancy in the falling points of the sheets on the ejection tray, thereby worsening the registration of the sheets collected on the ejection tray.
Incidentally, some image forming systems are of the type that the sheets can be sorted in such a manner that they are shifted (when out of position) in groups on the ejection tray by shifting a pair of ejection rollers in the axial direction or by shifting the ejection tray in a direction perpendicular to a sheet feeding direction. Particularly, in an image forming system of this kind, in order to maintain the sorted condition, the sheets must be collected on the ejection tray with correct registration.
For this reason, in the image forming system having a deep ejection tray 2 attached to an upper part of a body frame 1 as shown in FIG. 4, there is normally provided a sheet hold-down member 5 for urging or holding down a leading end of the sheet (not shown) before a trailing end of the sheet falls on the ejection tray in order to prevent the movement of the sheet (on the ejection tray) ejected out of the system by means of a pair of ejector rollers 3.
The conventional sheet hold-down member 5 arranged at the ejection tray portion 2 of the image forming system is pivotally mounted, at its base end, on the body frame 1 via a support shaft 6 for pivotal movement in a direction (shown by the arrow) perpendicular to a surface of the tray, so that the sheet hold-down member can hold the sheet ejected on the ejection tray with a predetermined urging force by its own weight. In this case, the leading end of the sheet ejected out of the system by means of the ejector roller pair 3 is slidingly moved upwardly along the inclined surface of the tray and then is slid below a bent portion 7 formed on a free end of the hold-down member 5 so that the sheet is held down by the bent portion 7 with the predetermined force.
As mentioned above, although the sheet hold-down member holds the sheet by its own weight, if the weight of the member is too great, the leading end of the sheet cannot be slid below the bent portion 7, with the result that the sheets are not ordered on the tray, thus preventing the correct stacking of the sheets. Accordingly, the sheet hold-down member 5 is made of a thin molded plate or a fine metal wire or rod to reduce the weight thereof.
However, since the conventional sheet hold-down member 5 arranged at the ejection tray portion 2 of the image forming system is weak in construction, it is easily deformed or damaged if an operator roughly handles it, with the result that the correct or normal function of the sheet hold-down member will deteriorate. Further, since the sheet hold-down member 5 is pivoted around the support shaft 6, as it is lifted in accordance with the increase in the sheets collected on the ejection tray, the urging force of the sheet hold-down member acting on the sheet will be decreased, thus making the sheet holding function unstable.
On the other hand, in the image forming system having the deep ejection tray 2, since a large number of sheets can be collected, the operator does not often pay attention to the sheet ejecting condition in comparison with normal image forming systems, and, thus, in many cases, a sheet full load condition is left as it is. Consequently, overejected sheets sent to the ejection tray exceeding its maximum sheet collecting capacity are dropped onto a floor to become dirty or are jammed in the vicinity of the ejector roller pair 3.